The present invention relates to a system (apparatus and methods) for treatment of waste water streams; more particularly to a system for the primary treatment of waste water streams, including grit removal, flow equalization, fine screening, flow measurement, and primary clarification; and most particularly to a system for performing such functions in a single primary settling tank, wherein the tank is made to be self-flushing of settled solids on the tank walls and floor by the hydraulic force of the influent waste water stream.
Prior art waste water treatment systems used in the industry generally include, but are not limited to, the following treatment processes: grit removal, fine screening, flow equalization, and primary clarification. The typical treatment processes are dependent on the velocity at which the waste water is moving through the system. Waste water, however, is not produced continually by humans, but instead is created in batch type processes, such as showering, flushing a toilet, or operating a washing machine. Such water consumptive activities are generally repetitive, resulting in daily, weekly, monthly, and yearly diurnal flow patterns for a specific waste water treatment system.
Grit removal is generally performed early in the treatment process in a grit chamber which is velocity sensitive having a design velocity of 1.0 to 3.0 feet per second. The most common method for removing grit is gravitational by reducing the velocity of the influent flow so that the grit settles out, or centrifugally by utilizing a circular channel/tank. The circular channel/tank is a hydro-cyclone that causes the grit to settle in a sump, separating the grit so that the organics can move forward to the biological processes. The grit is then pumped out of the sump to a grit washer and discharged to a dumpster for disposal at a landfill. This technology is subject to a large variation in influent flows.
Fine screening is typically accomplished by placing a screen in an influent channel. The influent channel must have a minimum velocity of 1.25 feet per second to keep solids from settling out in the channel and a maximum velocity of 3.0 feet per second to keep solids from being forced through the screen. Such a flow is difficult to maintain due to the large variation in diurnal and pumped flow patterns.
Primary clarifiers are also velocity sensitive having design requirements of 1,000 gallons per day per square foot and a minimum depth of 10 feet with the heavy solids settling to the base of the clarifier where they are pumped to a digester, the floatable solids, grease and scum being trapped and skimmed off the surface and the neutral buoyant solids/clarified wastewater exiting the basin via an effluent weir. Primary clarifiers are typically large tanks designed for gravity settling and may include electrical drives, flights and chains, rack arms, and paddles or suction tubes and sludge pumps.
Flow equalization typically occurs in a separate tank. The flow at the waste water plant is subject to travel times in the collection system, collection system design, and pump station sizing. In general, larger collection systems use pump stations to lift the waste water to the treatment facility. The pumps are typically placed on variable-frequency drives in an attempt to provide a consistent flow. The system of variable-frequency drives and pumps, however, fails in low- and high-flow conditions. The pumps must be designed for peak hourly flows and centrifugal pumps have minimal turn-down capabilities.
These treatments and processes each require their own mechanical equipment and supporting infrastructure, resulting in higher capital costs to develop the system, increased maintenance, and higher energy costs. Further, the necessary supporting infrastructure requires a large amount of space to develop a traditional waste water treatment system. Thus, there is a need to develop a waste water treatment system wherein the treatment processes are performed in a single tank, which uses less energy, requires fewer construction materials, and takes up less space, while at the same time offering improved BOD reduction and a uniform flow to downstream processes.
Parent U.S. patent application “Primary Equalization Settling Tank”, Ser. No. 12/177,553, now U.S. Pat. No. 7,972,505, discloses method and apparatus for providing these primary treatment functions. The system comprises a single primary settling tank that performs grit removal, flow equalization, primary clarification, and fine screening. A flow measurement device is located on the discharge side of the primary settling tank to measure the actual effluent flow from the primary settling tank. Further, an air blower may be provided to aerate the wastewater in the primary settling tank to prevent the wastewater from becoming septic.
The prior art wastewater treatment system also includes at least one sludge withdrawal hopper, a scum trough, a fine screen, and a discharge mechanism. In one embodiment, the discharge mechanism includes a driven decanter, which operates on a pivot and has a maximum operating height of one foot below the wall of the primary settling tank. The fine screen may be attached directly to the intake of the driven decanter. The height of the driven decanter is adjustable to match the actual effluent flow to a target flow by varying immersion of the decanter in the tank. The driven decanter may further include an actuated valve and a source of compressed air to pressurize the driven decanter to defoul the fine screen when the actuated valve is closed. The driven decanter may further include a source of potable or non-potable water to backwash the fine screen when the actuated valve is closed.
In another embodiment of the prior art invention, the discharge mechanism includes a floating decanter. The fine screen may be attached directly to the intake of the floating decanter, which is attached to a discharge pipe by a flexible hose. The discharge pipe includes an actuated valve, adjustable to match the actual effluent flow to a target flow. The discharge pipe may also include a source of compressed air ahead of the actuated valve to pressurize the driven decanter to defoul the fine screen when the actuated valve is closed. In a further embodiment, the discharge pipe may include a source of potable or non-potable water ahead of the actuated valve to backwash the fine screen when the actuated valve is closed.
In a further embodiment of the prior art invention, the discharge mechanism includes a fixed discharge pipe, and the fine screen is mounted vertically around the discharge pipe.
The '553 prior art disclosure further provides a method for treating wastewater in a single primary settling tank, comprising the steps of grit removal, fine screening, flow equalization, and primary clarification. The method further comprises using a flow measurement device on the discharge side of the primary settling tank and adjusting a discharge mechanism operationally connected to the flow measurement device to match the actual effluent flow rate to a target flow rate.
In the prior art system, the influent is discharged into the tank at a single, unspecified point, and no provision is made for distributing the influent within the tank. Thus, solids can build up non-uniformly on the tank walls and the sloping tank bottom. Further, no mechanical or hydraulic means is providing for assisting movement of accumulated solids toward the sludge trough at the bottom of the tank. Thus, removal of solids from the tank walls and bottom can require significant operator input.
What is needed in the art is an improved self-flushing system for a waste water primary equalization settling tank wherein waste water influent is distributed laterally and substantially uniformly across the width of the tank; wherein grit and other solids are removed by settling at a plurality of locations in the process; and wherein the gravitational hydraulic force of waste water influent is employed to flush accumulated solids from wall and bottom surfaces of the tank toward a sludge trough at the bottom of the tank.
It is a principal object of the invention to improve the self-flushing capability of a primary waste water treatment tank.